I have the moments in the form of indices
Why bother ? .. if you are doing the sums on a PC, stay in moment .. using IU just adds calculations for the sake of adding calculations and certainly gives no benefit along the way. I've not done any load work on Airbus but I can't imagine that they would do things differently .. then again .. ?
By dividing your total index (sum of all indicies, which will be in the form of KGM, Foot-pounds or whatever system you are using) by your all up weight, you end up with a distance.
Suggest that either
CG = total moment/total weight, or
CG = (total index * index constant)/total weight
You therefore meed to know where the datum is
Very true. Generally the FS datum is near, or forward of, the nose .. such a bore .. negative numbers. For graphical loading system work, it is more usual to see the (trim) datum somewhere inside the CG envelope.
MAC is simply the average chord
Not quite .. MAC gives a theoretical wing with similar aerodynamic properties